A superconductor is a perfect conductor that shows no energy loss at a very low temperature since an electric resistance is completely disappeared, and it is applied to various fields. Recently, a power cable employing a superconducting wire is used for meeting the increasing power demands in a metropolis.
The power cable employing a superconducting wire shows better electricity transmission capability than a conventional one employing copper, so this power cable may transmit a large amount of power and reduce losses during electric transmission rather than a power cable employing copper. Thus, there is no need to increase a sectional area of a cable for transmission of a large amount of power, and therefore the above power cable may efficiently transmit a large amount of power though the cable is designed in a small size.
Meanwhile, a superconductor may keep its superconductivity only within the ranges of three critical values, namely a critical temperature, a critical magnetic field and a critical current density, so the superconductor loses the superconductivity and changes its phase into an ordinary conductive state beyond the critical values. Thus, it is required to check whether the superconductivity of a superconducting wire is kept during the operation of the superconducting power cable.
FIG. 1 is a longitudinal sectional view showing a conventional superconducting power cable having a quench detection structure of a superconducting conductor layer. Referring to FIG. 1, the superconducting power cable includes a former 1, a conductor layer 2 made of a superconducting wire and surrounding the former 1, an insulating layer 3 for insulating the conductor layer 2, and a shielding layer 4 surrounding the insulating layer 3 for electric shielding. A flattening layer (not shown) interposed between the former 1 and the conductor layer 2 and surrounding the former 1 may be further included so as to flatten the surface of the former 1.
The conductor layer 2 is configured with a superconducting wire having a tape shape arranged on a circumferential surface of the former 1, and it has a multi-layer structure for transmitting a large amount of power. The superconducting wire frequently loses its superconductivity since it comes out of the above critical ranges due to a defect of the superconducting wire itself, a defect capable of being generated during the cabling procedure, and a defect of a cooling system caused by impurities included in a coolant that keeps the superconducting wire below a critical temperature. This phenomenon that a superconductor loses its superconductivity and changes its phase into an ordinary conductive state is called quench.
If quench is generated in any superconducting wire of the superconducting power cable to change the phase of the superconducting wire into an ordinary conductive state, an amount of current capable of flowing in the superconducting wire is decreased. Thus, though a superconducting power cable is manufactured to have a constant electric current flowing in each layer of the superconducting conductor layer 2, if quench occurs, remaining electric currents flow in other superconducting wires where no quench occurs. As a result, the possibility that other superconducting wires having no quench exceed a critical current is increased. If quench occurs in another superconducting wire exceeding the critical current, the superconducting state of the cable itself is broken, so the transmission capability of the cable is greatly deteriorated.
Thus, it is important to detect the generation of quench during the operation of the superconducting power cable. For this purpose, voltage terminals 5 provided to both ends of the conductor layer 2 in a length direction may be used to detect quench by measuring voltages of both ends. However, since a high voltage is applied to the conductor layer 2 of the superconducting power cable, it is impossible to measure minute signals generated by quench. Even if this measurement is available, a measuring device is damaged due to the high voltage, so quench cannot be detected while an electric power is actually transmitted.